Well, I too think that the bootloader-entering-combination for the updated 4.1.2 build is not well documented as I searched multiple minutes for it when I needed it

The key is: Power the phone on with Power + Volume Down, release the power button instantaneously as the phone vibrates and hold volume down until the bootloader is there.
This also works on any reboot: Just press and hold the volume down button shortly before the device starts to vibrate (see the green light) - no need to press the power button there.

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